The present invention is concerned with agglomerates obtained from organic waste (such as sewage sludge solids or refuse derived fuel), which are suitable for use in a gasification process or as a combustible fuel for use in a boiler or the like.
Sewage sludge is the insoluble part of raw sewage, which may be present in the residue left after aerobic or anaerobic treatment of sewage; current methods of disposal of sewage sludge include landfill and dumping at sea, neither of which is satisfactory from an environmental point of view. Difficulties are encountered with incineration of sewage sludge, because of the high water content of sewage sludge, the presence of toxic heavy metals, and the fact that safe incineration would require flame temperatures in the region of plasma temperatures.
However, various processes have been recently proposed for the production of fuel gas from waste materials, including sewage sludge. Typical such processes, for example, as disclosed in U.S. Pat. No. 5,125,931 and WO94/17161, involve mixing dewatered sewage sludge with crushed coal and comminuted cellulosic waste solids in defined ratios and water contents, pressing the resulting mixture into briquettes or pellets, and gasifying the briquette by reaction with oxygen and steam to produce a mixture of hydrogen and carbon monoxide.
Among the binders disclosed for such briquettes are molasses, black liquor, lignin sulfonate derivatives, brewery wastes, starch wastes, bentonite, slaked lime, unslaked lime, bitumen and pitch. No indication of preference is given for any of these binders, and no conditions are given for use of these binders.
Briquettes based on organic waste materials, such as those described above, may be gasified to produce a fuel gas, which can be burnt in a gas turbine, followed by exhaust gas recovery to generate steam, which in turn can be used to drive a steam turbine, both turbines being connected to generators to generate electricity.
For satisfactory commercial use in such a gasification process, the briquettes must meet the following criteria:
(a) they must have sufficient physical strength (cold green strength), in terms of crush strength, shatter resistance and abrasion/attrition resistance, for them to survive transportation, handling and feeding to the gasification process without disintegrating; PA0 (b) they must maintain their structural integrity in the gasifier as they flow down the fuel bed under gravity (that is, they should not disintegrate or form dust as they pass through the various reaction zones of the gasifier, including the drying, devolatilisation, pyrolysis and gasification zones); PA0 (c) they should be resistant to thermal shock (that is, they should be capable of withstanding instantaneous exposure to high temperatures, which, for demonstration purposes, may be those in an oven at about 800 degrees Celsius, without disintegration or significant loss of structural integrity); PA0 (d) they should have good carbonization properties, that is, they should be amenable to gasification; PA0 (e) they should not stick together, in order to avoid blocking of the gas flow and to ensure maintenance of bed voidage (the inherent porosity of the briquettes themselves is also helpful in the various reactions they undergo in the gasifier); and PA0 (f) in the case of a slagging gasifier, the ash content should form a molten slag pool, which can be tapped intermittently and quenched to give a solid vitreous frit which encapsulates the heavy metals and the like.
The above criteria must be met within the financial constraints of a commercially viable process.
We have now devised a method of making improved briquettes and other agglomerates, which involves the use of a specially selected class of binder, such that the resulting agglomerates readily meet the above criteria, and are selectively suitable either for use in a gasification process as described above, or in a combustion process.